The Legality of Migrant Detention on Military Bases

There is mounting evidence that the Trump administration will make substantial use of military bases to detain up to tens of thousands of migrants. The public has been told, for example, that the Army is planning to house as many as 30,000 migrants on Army installations. It also appears that the Department of Homeland Security (DHS) has already sent a request, of indeterminate scope, to the Defense Department for housing support. And the Trump administration is actively preparing to invoke the Alien Enemy Act to quicken the pace of deportations.

The administration appears to be contemplating three avenues to justify migrant detention on military installations: first, as a form of interagency support provided to DHS under the Economy Act; second, as a mission of the Defense Department, carried out in furtherance of President Trump’s executive orders; and third, under the authority of the Alien Enemy Act. Only the first legal theory has a solid foundation in the law, past executive branch practice, and legal precedent.

Before proceeding to these three theories, it’s important to establish two baseline principles. First, the Posse Comitatus Act prohibits using the military as “a posse comitatus [a group of people who are called upon to suppress lawlessness] or otherwise to execute the laws.” What does it mean to execute the laws? Three tests have emerged from the case law, all of which the Department of Justice Office of Legal Counsel (OLC) has adopted. The court in United States v. Red Feather, for example, held that the act prohibits “the direct active participation of federal military troops in law enforcement activities.” In United States v. Bacon, the U.S. Court of Appeals for the Eleventh Circuit instead found that the act prohibits the military from undertaking any activities that pervade the activities of civilian [law enforcement] officials.” Still other courts have defined the scope of the act as any military activity that is “regulatory, proscriptive, or compulsory in nature and causes the citizens to be presently or prospectively subject to regulations, proscriptions, or compulsions imposed by military authority.”

However defined, using the military for detaining civilians certainly violates the act. The Posse Comitatus Act itself recognizes only two exceptions to this blanket prohibition—express authorizations for law enforcement activity undertaken pursuant to statutory or constitutional authority. As OLC reaffirmed at the end of the first Trump administration:

[F]or a statute to authorize a military activity expressly for purposes of the Posse Comitatus Act, we think that it must be clear that Congress has approved that activity without regard to the restrictions on using the military “as a posse comitatus or otherwise to execute the laws.” … We do not think that a general authorization of “assistance” necessarily means that the restrictions of the Posse Comitatus Act fall away.

This approach is not new. In 1878, Attorney General Charles Devens opined that the Judiciary Act of 1789’s authorization for U.S. marshals to “command all necessary assistance in the execution of [their] duty” was not an “express authority” for marshals to “summon any military force of the United States.” Attorneys general affirmed this understanding of the act in 1881, 1889, and 1957. The Defense Department’s list of statutory exceptions to the act bears out this approach—for a statute to constitute an express exception to the act, it must specifically contemplate using the military for a law enforcement activity.

The second baseline principle is that the president cannot sidestep this statutory restriction by changing the functions of the Defense Department—such as, for example, declaring that immigration enforcement is a Defense Department mission. It has long been the position of the executive branch that the president may not reallocate the statutory responsibilities of one department or agency to another. OLC said as much in 2002, as the Bush administration was looking for ways to better coordinate homeland security responsibilities after the 9/11 attacks. Pointing to the Necessary and Proper Clause, OLC argued that “Congress may prescribe that a particular executive function may be performed only by a designated official within the Executive Branch, and not by the President.” And it concluded that “the President may not transfer the statutory duties and functions of a bureau in one Cabinet department to another Cabinet department without an act of Congress.”

This was far from a novel interpretation of the Constitution. In 1823, Attorney General William Wirt (our ninth attorney general, across the Monroe and Quincy Adams administrations) concluded that:

[i]f the laws ... require a particular officer by name ... perform a duty, not only is that officer bound to perform it, but no other officers can perform it without a violation of the law; and were the President to perform it, he would not only be not taking care that the laws were faithfully executed, but he would be violating them himself.

He affirmed this opinion the next year. OLC came to the same opinion in 1980, finding that the Department of Justice could not transfer its litigating authority to another agency because:

[i]n the absence of any general provision of law permitting an agency to transfer its statutory authority to another agency, such transfers or delegations may normally be accomplished only by legislation.

Avenues to Migrant Detention on Military Bases 

The Economy Act

The first, most legally defensible, theory justifying Defense Department assistance with migrant detention is that it provides reimbursable support to DHS under the Economy Act. As I discussed previously on Lawfare, the Economy Act authorizes one agency to provide goods or services to another on a reimbursable basis. Enacted in 1932, the act was meant to incentivize agencies to contract with each other for goods and services. Often, one agency is equipped to assist another agency in executing that other agency’s statutory functions at a lower cost. Economy Act agreements are the bread and butter of interagency cooperation. General requirements regarding Economy Act agreements are provided in 48 CFR 17.5 and include, for example, that the use of interagency support is in the best interest of the government and that the goods or services cannot be acquired “as conveniently or economically by contracting directly with a private source,” among other conditions.

Most relevant to prospective detention of migrants on military installations, both during the early days of the coronavirus pandemic and for many months during and after the evacuation from Afghanistan, the Defense Department housed nonmilitary personnel on military installations under the Economy Act. This type of support begins with a request for assistance transmitted from one agency to another. DHS has transmitted a request to the Defense Department to assist with housing migrants, which would suggest that the administration plans to operate under the terms of the Economy Act. But the public knows next to nothing about the scope or legal contents of the request. This is, in itself, noteworthy. During the coronavirus pandemic and after the evacuation from Afghanistan, the Defense Department clearly stated that its support was provided on a reimbursable basis. There has been no comparable transparency and little in the way of journalism or congressional inquiry to clarify what is going on in the current situation.

Clearly, the Economy Act does not authorize using the military for law enforcement purposes in a manner that would constitute an exception to the Posse Comitatus Act. And unlike the Reorganization Act (discussed below), it does not affect at all the division of responsibilities between the Defense Department and DHS.

Executive Orders

This raises the second theory that may be at play, one that is legally indefensible—that the president’s executive orders are sufficient legal basis for the Defense Department to support DHS’s migrant detention. Section 1 of Trump’s declaration of a national emergency at the southern provides that:

[t]he Secretary of Defense, or the Secretary of each relevant military department, as appropriate and consistent with applicable law, shall order as many units or members of the Armed Forces, including the Ready Reserve and the National Guard, as the Secretary of Defense determines to be appropriate to support the activities of the Secretary of Homeland Security in obtaining complete operational control of the southern border of the United States. The Secretary of Defense shall further take all appropriate action to facilitate the operational needs of the Secretary of Homeland Security along the southern border, including through the provision of appropriate detention space, transportation (including aircraft), and other logistics services in support of civilian-controlled law enforcement operations.

Without specific statutory authority providing otherwise, this directive cannot itself transfer responsibility for immigration enforcement from DHS to the Defense Department. Border security is, by statute, a responsibility of DHS: 6 U.S.C. § 202 provides that the secretary of DHS is responsible for “preventing the entry of terrorists and the instruments of terrorism into the United States,” “securing the borders, territorial waters, ports, terminals, waterways, and air, land, and sea transportation systems of the United States,” and carrying out immigration enforcement. As noted above, this is exactly the kind of statutory duty that the executive branch has long understood requires a statutory authority to shift to another department or agency.

Congress once provided the president a greater degree of flexibility in shifting agency responsibilities under the Reorganization Act of 1939. This statute authorized the president to transfer the functions of one agency to another after investigation, provided that a detailed process and enumerated considerations were met. Importantly, the act’s authority expired in 1984.

Not only has the executive branch long recognized that the president cannot commit the statutory responsibilities of one agency to another, but Congress has in the past specifically provided such an authority to the president—an authority that is no longer available.

The Alien Enemy Act

Finally, the Defense Department is responsible for homeland defense, which brings us to the final legal theory that could be used to justify migrant detention—the Alien Enemy Act. Many others have written in great depth about the reasons why invoking the Alien Enemy Act with respect to drug cartels would be contrary to the text and historical uses of the act. To summarize, there are two primary issues with meeting the requirements of the statute.

First, the powers made available by the act are available during “a declared war between the United States and any foreign nation or government” or when a foreign nation or government invades or commits (or attempts or threatens to commit) a “predatory incursion” against the territory of the United States. The statute does not define “foreign nation or government.” Other provisions of the U.S. Code define “foreign government” broadly (see, for example, 18 U.S.C. § 11). But those definitions have no bearing on the meaning of “foreign nation or government” for the purposes of the Alien Enemy Act. Instead, we must look to past presidential invocations of the act. As Ania Zolyniak has noted, all were in the context of congressionally declared wars against already-recognized nation-states.

Second, the act may be invoked only in response to an invasion or predatory incursion. Again, all invocations of the Alien Enemy Act have occurred in the context of a congressionally declared war. Ilya Somin has also argued that founding-era understandings of “invasion” took the term to mean an organized armed attack. There is little reason to believe that, factually, drug or human smuggling across the border is equivalent to such an organized, armed attack.

Interestingly, the Alien Enemy Act does not specifically authorize using the military for domestic law enforcement and, therefore, is not a statutory exception to the Posse Comitatus Act. Compare its terms to, for example, 16 U.S.C. § 78, which authorizes the secretary of the Army to “make the necessary detail of troops to prevent trespassers or intruders from entering the Sequoia National Park, the Yosemite National Park, and the General Grant National Park, respectively, in California, for the purpose of destroying the game or objects of curiosity therein.” The executive branch has recognized that the Alien Enemy Act isn’t an exception to the Posse Comitatus Act—the Defense Department, for example, does not list it as such in the Defense Department regulation that implements the act.

This makes sense because the Alien Enemy Act is, properly understood, a wartime authority applicable in the context of military operations. It simply does not pertain to the day-in, day-out civilian law enforcement that the Posse Comitatus Act is meant to prohibit the military from undertaking.

Executive branch interpretations of the Posse Comitatus Act underscore this distinction. First, as I have mentioned previously on Lawfare, since 1977, OLC has argued that the Posse Comitatus Act does not restrict the military’s ability to apprehend civilians for violating federal or state law on a military installation when the activities “threaten the security and good order of the base.” This would explain, for example, reporting that anticipates active-duty military personnel will provide perimeter security around the portion of the base where migrants may be housed.

Further, Defense Department regulations provide that the Posse Comitatus Act does not restrict “[a]ctions taken for the primary purpose of furthering a [Defense Department] or foreign affairs function of the United States, regardless of incidental benefits to civil authorities.” These functions “may include,” among others, “[s]uch other actions that are undertaken primarily for a defense purpose.” Elsewhere, OLC has opined that this exception includes any military activities taken outside the United States and in areas controlled by the military pursuant to the laws of war.

Whatever we think of these practice-based exceptions to the Posse Comitatus Act, they underscore why the executive branch has not needed to stretch the Alien Enemy Act’s authority to construe it as an exception to the Posse Comitatus Act. The Alien Enemy Act is a wartime authority—authorizing, in OLC’s words, “actions taken for the primary purpose of furthering a [Defense Department] or foreign affairs function of the United States.”

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There are signs that these historically accepted bounds of the Posse Comitatus Act continue to hold sway within the Defense Department. Reporting has indicated, for example, that the only military personnel slated to directly detain migrants will be Texas National Guard who are “federally deputized.” Although unstated, this appears to recognize that the only way to use the military to directly detain migrants is through a now increasingly recognized, significant loophole to the Posse Comitatus Act—32 U.S.C. § 502(f). Under this mobilization authority, the president or secretary of defense can request that governors provide members of their National Guards to perform “other duty” in support of a federal military mission. In this duty status, National Guard personnel remain part of their state and territorial militias and, therefore, are not subject to the Posse Comitatus Act’s restrictions.

As I have previously written on Lawfare, for many decades the executive branch has used the National Guard in this duty status to work around the prohibitions of the Posse Comitatus Act, including for deployments at the southern border. While I believe that such a significant loophole to the Posse Comitatus Act is not desirable, the fact that it appears to still be relevant suggests continuity in the Defense Department’s interpretation of the act.

The Trump administration is entertaining a variety of implausible legal theories in its drive to use the military to expedite deportations. This does not mean that there are no legal routes to doing so. Indeed, Congress has committed to the president and secretary of defense great discretion through statutory law. The days and weeks ahead will reveal the administration’s appetite for throwing off even these minimal shackles on its ability to use the military within the United States.